In the usual process for continuously stretching a thermoplastic band, strip, web, foil, or film the web to be stretched is fed into the stretching apparatus at a predetermined relatively low input speed. In the apparatus it is heated sufficiently to soften it somewhat, and it is displaced out the outlet side at a relatively high outlet or stretching speed. The resultant tension in the web combined with the softening of this web causes it to stretch longitudinally and improves many characteristics of the web, for instance its strength, gloss, and transparency.
The web stretches in the stretching apparatus down-stream from a so-called stretch point which is that region in the web where the resistance to plastic (not elastic) deformation is first less than the tension in the web. Thus the location of this stretch point is determined by many rheological factors including but not limited to the type of resin, temperature of the web, and tension in the web, which last-named effect is largely a function in turn of the difference between the input and output speeds.
With the known systems, therefore, this stretch point wanders considerably while the actual stretch imparted to the goods will in fact be determined wholly by the difference between the input and output speeds and will not change even when the stretch point moves. Thus this amount of stretch will be imparted to the web over a short or long distance, depending on whether the stretch point is further upstream or downstream relative to the longitudinal advance direction of the web.
Such a wandering stretch point produces a finished workpiece of varying physical characteristics as some parts of the web might be heated to the softening point more rapidly than others and some parts might be subjected to stretching for longer or shorter times. When the stretch is imparted over a too short distance the thickness, transparency, and other characteristics of the web may be affected adversely, and the web in fact may be torn. When stretching takes place over a too long distance the same and other characteristics may also be deleteriously affected.